1. Field of the Invention
This invention relates generally to the field of gas exploders adapted to rest on the earth and deliver one or more seismic compressional pulses into the earth. The invention is more particularly concerned with exploders of this character which are fired repeatedly at very short intervals.
2. Description of the Prior Art
Seismic gas exploders or as they are sometimes termed, seismic wave generators, broadly speaking, include a detonation or combustion chamber having a rigid bottom adapted to rest on the earth and a rigid top telescopically or otherwise joined therewith in a suitable fashion to permit relative vertical movement therebetween. Initiation of the explosion of a combustible fuel gas and oxidizing gas mixture within the chamber produces the desired relative movement and applies a compressional pulse to the earth through the rigid bottom of the chamber.
Normally, fuel gas and oxygen from separate containers are mixed prior to being conducted into the detonation chamber in correct proportions by means of flow controlling solenoid valves interposed between the separate containers and the mixing point. Upon closing of the solenoid valves, the combustible gas mixture is ignited by a spark source located at the point of mixing. The resulting combustion accelerates into a detonation front within the inlet pipe or conduit leading to the exploder's detonation chamber.
The heat generated by the chemical reaction involved in the burning and detonation of such a gas mixture does not of course dissipate immediately. If a gas exploder is fired repetitively at a rapid rate, one of the barrier problems is ignition of gases during fill prior to activation of the spark source, i.e. preignition. The higher the rate of fire, the sooner this limitation is manifested. In practicing modern seismic data acquisition techniques, it is desirable to be able to generate long pulse sequences, ranging, for example, up to 100 or 200 pulses. For this and other reasons, therefore, the problem of preignition has assumed greater importance.
Premature ignition may be the result of several factors including but not necessarily limited to: (1) residual heat stored in the walls of the exploder, inlet conduit or mixed-spark ignition assembly, (2) residual flame due to combustion of gases not burned in the detonation front (to be distinguished from failure of solenoid valves to close completely, resulting in a sustained flame), (3) heat released by carbon in gas form converting to solid form, (4) glowing particles adhering to walls of the exploder and inlet, and (5) hot spots at localized points due to detonation and subsequent shock waves.
The prior art has recognized the existence of the preignition problem; but so far as Applicant is aware, it has assumed that among the factors mentioned above the main contributor has been the buildup of heat within the detonation chamber itself. Thus, for example, exhaust valving arrangements have been devised more efficiently to release the spent gases from the explosion. This approach has met with little success.
Applicant has, by contrast, focused his attention outside the detonation chamber. It is hypothesized that since the flame or combustion front is initiated in or adjacent to the external mixer and accelerated therefrom toward the detonation chamber, the "dwell" time of the moving front is necessarily greatest in the external mixer and the connective tubing. It is the Applicant's conviction, based upon experimental evidence to date, that the buildup of residual heat in these external components plays a much greater role than heretofore suspected.
It was apparent, therefore, that significant advantage in the number of shots obtainable from a repetitively fired seismic gas exploder should be possible if the utilization rate of an individual gas inlet and associated external elements were reduced, or, in other words, if cooling time were increased. In order to maintain a given repetition rate for the exploder, it was decided to modify the conventional charging and ignition system to accomplish this result.